The theory and principles of use of MOSFETs for detection of radiation is well known. It is understood that the threshold voltage of a MOSFET or, as it is sometimes called an insulated gate field effect transistor, (IGFET) varies with exposure to radiation and therefore provides a useful building block in the design of dosimeters.
On Nov. 20, 1984 U.S. Pat. No. 4,484,076 issued with Ian Thomson as inventor. The patent discloses a metal oxide semiconductor device, the threshold voltage of which provides the accumulated dose at any particular period of time. A differentiator is also used to provide the dose rate. Canadian Patent 1,204,885 issued May 20, 1986 to the same inventor. It describes a further advanced dosimeter using two MOSFETs and measuring the difference in the threshold voltages of the transistors. This value is indicative of the total accumulated dose of radiation to which this device was exposed. This provided a significant advance in the art in that the differential technique lessened the deleterious effects relating to stability common to both transistors.
The inventor and M. H. Reece in a paper entitled Semiconductor MOSFET DOSIMETRY published in the proceedings of the Health Physics Society 1988 Annual Meeting present the structure and operation of MOSFET dosimeters. The paper discloses experimental results with respect to response to radiation of different kinds and discusses application of such devices. The paper provides useful background information for this application.
Canadian Patent No. 1,204,885 issued May 20, 1986 again with Ian Thomson an inventor, discloses a direct reading solid state dosimeter which will measure neutron and gamma radiation. A dual metal oxide semiconductor field effect transistor (MOSFET) is used as a gamma sensor and a forward biased PIN diode as a neutron sensor. A liquid crystal display is provided for displaying the gamma and neutron radiation dosage. The basic operation of the circuit is effect by insulated gate field effect transistors, having their drains connected together. A battery or some other device provides bias potential between the gate, drain, and source of the first transistor, while the gate and drain of second transistor are connected together.
Another patent in the dosimeter field is East U.S. Pat. No. 4,757,202 dated Jul. 12, 1988. This patent discloses a dosimeter comprising a dual MOSFET as a gamma sensor and a forward biased PIN diode as a neutron sensor. A liquid crystal display is used to display the accumulated dose.
There are a number of applications that require ionizing radiation dose to be measured to a high degree of accuracy. Consequently, the radiation dosimeters which are used to make these measurements have to exhibit a high degree of accuracy and stability, especially where there is a wide variance in ambient temperature. An example of this is the use of a personal dosimeter for monitoring fire fighters and police who have to work in or near accidental radiation environments. Those dosimeters should have a sensitivity of approximately 0.010 cGy(rad). In an accident situation, a nuclear worker is allowed up to 1 cGy per body dose. The accumulated dose over a period of time is also a critical measure because Federal regulations state that the maximum body dose allowed over a one year period is 5 cGy.
Another example is in the area of radiotherapy, where dosimeters are used to calibrate the beam profiles of radiation treatment equipment and to monitor patients exposed to doses of radiation. These instruments must therefore be stable to within 5%. Typically, doses range from a few cGy to a few thousand cGy.
A third application is in military personal dosimeters, which are required to have a range of 1 cGy to 2000 cGy with re-zeroing capabilities. They should also be stable over the temperature ranges of operation which typically varies from -20.degree. to +50.degree. celsius.
A technique of measuring radiation dose makes use of an insulated gate field effect transistor (IGFET) or a MOSFET as otherwise commonly known, as a radiation sensor. Radiation causes a shift in the threshold voltage of the IGFET. This change in threshold voltage is measured and represents the radiation dose received.
In its basic form, this type of prior art dosimeter consists of two silicon insulated gate field effect transistors which share the same substrate, each having a gate, a source and a drain. A differential threshold voltage between the transistors is measured and stored. The gate of the first transistor is biased positively with respect to its source, drain and substrate, while the gate of the second transistor is held at a slightly decreased bias with respect to its own source, drain and substrate. The device is now sensitive to ionizing radiation and will respond when exposed. A differential threshold voltage is measured again between the transistors and stored. The difference between the threshold voltages indicates the measure of the radiation dose.
IGFETS, however, exhibit a number of problems which limit their use in personal and/or radiotherapy dosimeters. In switching on a P-channel IGFET, there occurs a small threshold voltage drift. Typically a threshold drift has been measured at 30 mV, maximum, in unradiated devices. This phenomenon is associated with the slow surface states located at the silicon/silicon dioxide interface of the IGFET. This effect is particularly pronounced in highly irradiated devices and can be in the order of 500 mV (corresponding to 500 cGY or greater).
Though these slow surface states effect both IGFETS in the same manner, there still exists a differential threshold voltage drift. This shift can be as low as 10 mV (corresponding to 10 cGy), and as high as 122 MV (122 cGY) depending on the radiation history of the dual device. This is an unacceptably low degree of stability for a low dose personal dosimeter, highly accurate medical instrument or laboratory dosimeter.
A second problem associated with the prior art is the difficulty of tracking two current sources with temperature. Separate current sources were used in the prior art dosimeters to provide a constant source to drain current to whichever one of the two IGFETs was being measured for threshold voltage. An approximate 1% change in the source to drain current results in a 20 millivolt shift in threshold voltage.
A third problem associated with the prior art is that the circuit does not allow for continuous reading capabilities. Emergency, military, and medical dosimeters should be continuous reading which gives the user the option of incorporating an alarm if desired. Such dosimeters also should have a rezeroing capability.
It is an object of this invention to provide a dosimeter for measuring ionizing radiation and particularly to a dosimeter having substantially improved accuracy and overcoming the problems of the prior art dosimeters as outlined above.